We all know that Honda has implemented and popularized the use of VTEC in their cars. I guess surely you would have heard this line at least once in your lifetime.. VTEC just kicked in yo.... Yeah... Without using a turbocharger, with VTEC itself they've managed to bring out more ponies. The same idea is also designed and implemented by Yamaha on their motorcycles. We can able to see this implementation in models like R15 version 3. We'll see about that later. Today in this post, let's see something about BMW's ShiftCam technology and how it works.
We know that BMW is known for its boxer engine. To produce more power and torque across every RPM band, BMW Motorrad's powertrain team has designed and developed this ShiftCam technology on their boxer engine in R 1250 GS and in S 1000 RR fitted with inline four cylinder.
Conventional valve train setup
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In conventional setup, only one cam lobe will be present for actuating the inlet valve. Valve lift and lift duration remains same in all RPM band. Either you partially open the throttle or opening it widely, same amount of fuel air mixture enters into the combustion chamber thus not making much change in the power output. To change this and to provide better power output in all RPM band, ShiftCam mechanism is developed.
Overview
This is designed to achieve both variable valve timing and variable valve lift. Here, two cam lobes will be present for a valve. One is low profile cam lobe and the other one is high profile cam lobe. So, totally there'll be four cam lobes present in the intake camshaft. At partial throttle condition or in lower RPM band, low profile cam lobe will be actuating the valves. In this condition, the cam lobe will be possessing shorter lift and lower duration which results in shorter time in opening of inlet valves. But when you twist your throttle wide open, you expect to feel the instant adrenaline rush. If low profile cams operate under the Wide Open Throttle (WOT) condition, you will not be feeling that instant adrenaline rush. To overcome this, ShiftCam technology comes into action.
To achieve more power in higher RPM band, the cam lobe should posses the characteristics of high valve lift and increased valve duration. This is where the second cam lobe comes to scene which is present next to the normal cam lobe. Both the cam lobe will be present side by side on the same shaft. The second cam lobe is designed in such a way to provide high valve lift and increased duration. But you may wonder how is it possible to change the cam lobes under running condition. This is taken care by the shift gate.
The camshafts are driven by the drive gear which is connected to the cam sprocket. In conventional valve train setup, the inlet and exhaust camshafts are directly driven by the timing chain which is directly connected to the crankshaft. But here, BMW motorrad has eliminated this system and implemented the drive gear mechanism. This drive gear rotates the intake and exhaust camshafts which is timed perfectly. Here, the thickness of the drive gear will be slightly higher compared to the thickness of the intake and exhaust camshaft's sprocket for a reason. At the end of the intake camshaft, shift gate will be present. Simply saying, the end of the intake camshaft is grooved.
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| When high profile cam lobe comes into action |
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| Valve's lift and lift duration under high profile cam lobe |
Near the shift gate, a electronic actuator will be present. The electronic actuator will push and pull the pin from it. The pin from the electronic actuator is designed to fit on the shift gate's groove. The Electronic Control Unit (ECU) controls the actuator by referring various factors like throttle position, engine's RPM and selected gear. When rider twists the throttle more, the ECU consider the above mentioned parameters and sends signal to the actuator. The signal makes the pin to come out from the actuator, sits and locks on the groove present in the shift gate. This action makes the camshaft to shift axially. When camshaft moves axially, high profile cam comes into contact with the valve head which results in high valve lift and more lift duration. Thus more fuel air mixture for banging the engine enters into the combustion chamber. The above mentioned characteristics of high valve lift and more valve lift duration helps in bringing out more power on high RPM band. Now you have understood why thickness of the drive gear is higher than the intake cam sprocket. When intake camshaft moves axially, the drive gear should give some space for acting like a splined shaft.
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| When return pin pulls the camshaft back to normal position |
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| Valve's lift and lift duration under low profile cam lobe |
When the throttle is closed, the return pin emerges from the actuator and pulls the camshaft back to the original position thereby the smaller cam profile comes into contact with the valve head for normal use. The whole above mentioned process happens in 10 milliseconds which is lesser than the time taken for the eye to blink.
When high profile cam is operating the valves, there'll be difference in opening of the two valves. This means, the inlet valves slightly opens at different time that is, one valve will be opening slightly slower than the other valve. This helps in creating a swirl motion of fuel air mixture while entering into the combustion chamber which helps in mixing. This also improves the combustion, making the mixture to burn fully thus proving a good fuel economy. It also helps in reducing the emission.
